0
点赞
收藏
分享

微信扫一扫

Hibernate -双向一对一关联关系映射

双向一对一关联关系映射中,关系数据模型有两种方式:按照外键映射和按照主键映射。

【1】基于外键映射的双向1-1

对于基于外键的1-1关联,其外键可以存放在任意一边,在需要存放外键一端,增加many-to-one元素。为many-to-one元素增加unique=“true” 属性来表示为1-1关联。

实例如下:

<!-- 使用 many-to-one 的方式来映射 1-1 关联关系 -->
<!-- name:deparment中关联的manager的属性名;
class:deparment中关联的manager的类型;
column:数据表DEPARTMENTS中拥有的外键列MGR_ID
unique:为外键列添加唯一索引
-->
<many-to-one name="mgr" class="Manager"
column="MGR_ID" unique="true">
</many-to-one>

另一端需要使用one-to-one元素,该元素使用 property-ref 属性指定使用被关联实体主键以外的字段作为关联字段。

实例如下:

<!-- 映射 1-1 的关联关系: 在对应的数据表中已经有外键了, 当前持久化类使用 one-to-one 进行映射 -->
<!--
没有外键的一端需要使用one-to-one元素,该元素使用 property-ref
属性指定使用被关联实体(department)主键以外的字段作为关联字段
-->
<one-to-one name="dept"
class="Department"
property-ref="mgr">
<!-- 属性引用通常为外键列(外键列对应的字段) -->
</one-to-one>

这里Department:Manager=主:从。

Department如下:

public class Department {

private Integer deptId;
private String deptName;

private Manager mgr;

public Integer getDeptId() {
return deptId;
}

public void setDeptId(Integer deptId) {
this.deptId = deptId;
}

public String getDeptName() {
return deptName;
}

public void setDeptName(String deptName) {
this.deptName = deptName;
}

public Manager getMgr() {
return mgr;
}

public void setMgr(Manager mgr) {
this.mgr = mgr;
}

@Override
public String toString() {
return "Department [deptId=" + deptId + ", deptName=" + deptName + ", mgr=" + mgr + "]";
}

}

Department.hbm.xml如下:

<hibernate-mapping package="com.jane.one2one.foreign">

<class name="Department" table="DEPARTMENTS">

<id name="deptId" type="java.lang.Integer">
<column name="DEPT_ID" />
<generator class="native" />
</id>

<property name="deptName" type="java.lang.String">
<column name="DEPT_NAME" />
</property>

<!-- 使用 many-to-one 的方式来映射 1-1 关联关系 -->
<!-- name:deparment中关联的manager的属性名;
class:deparment中关联的manager的类型;
column:数据表DEPARTMENTS中拥有的外键列MGR_ID
unique:为外键列添加唯一索引
-->
<many-to-one name="mgr" class="Manager"
column="MGR_ID" unique="true">
</many-to-one>

</class>
</hibernate-mapping>

Manager类如下:

public class Manager {

private Integer mgrId;
private String mgrName;

private Department dept;

public Integer getMgrId() {
return mgrId;
}

public void setMgrId(Integer mgrId) {
this.mgrId = mgrId;
}

public String getMgrName() {
return mgrName;
}

public void setMgrName(String mgrName) {
this.mgrName = mgrName;
}

public Department getDept() {
return dept;
}

public void setDept(Department dept) {
this.dept = dept;
}

@Override
public String toString() {
return "Manager [mgrId=" + mgrId + ", mgrName=" + mgrName + ", dept=" + dept + "]";
}

}

Manager.hbm.xml如下:

<hibernate-mapping package="com.jane.one2one.foreign">

<class name="Manager" table="MANAGERS">

<id name="mgrId" type="java.lang.Integer">
<column name="MGR_ID" />
<generator class="native" />
</id>

<property name="mgrName" type="java.lang.String">
<column name="MGR_NAME" />
</property>

<!-- 映射 1-1 的关联关系: 在对应的数据表中已经有外键了, 当前持久化类使用 one-to-one 进行映射 -->
<!-- 这里注意,如果配置在对象获取的时候会出问题 -->
<one-to-one name="dept" class="Department""></one-to-one>

</class>

</hibernate-mapping>

【2】代码测试

① 双向一对一持久化操作

代码如下:

@Test
public void testSave(){

Department department = new Department();
department.setDeptName("DEPT-BB");

Manager manager = new Manager();
manager.setMgrName("MGR-BB");

//设定关联关系
department.setMgr(manager);
manager.setDept(department);

//保存操作
//建议先保存没有外键列的那个对象. 这样会减少 UPDATE 语句
session.save(department);
session.save(manager);

}

如果先保存有外键的对象,则会多出update语句;如果先保存没有外键的对象,则不会有额外的update语句产生。

如下所示:

Hibernate: 
insert
into
DEPARTMENTS
(DEPT_NAME, MGR_ID)
values
(?, ?)
Hibernate:
insert
into
MANAGERS
(MGR_NAME)
values
(?)
Hibernate:
update
DEPARTMENTS
set
DEPT_NAME=?,
MGR_ID=?
where
DEPT_ID=?

② 双向一对一获取

代码如下:

@Test
public void testGet(){
//1. 默认情况下对关联属性使用懒加载
Department dept = (Department) session.get(Department.class, 1);
System.out.println(dept.getDeptName());

//2. 所以会出现懒加载异常的问题.
// session.close();
// Manager mgr = dept.getMgr();
// System.out.println(mgr.getClass());
// System.out.println(mgr.getMgrName());

//3. 查询 Manager 对象的连接条件应该是 dept.manager_id = mgr.manager_id
//而不应该是 dept.dept_id = mgr.manager_id
Manager mgr = dept.getMgr();
System.out.println(mgr.getMgrName());

}

如下所示:

Hibernate: 
select
department0_.DEPT_ID as DEPT_ID1_1_0_,
department0_.DEPT_NAME as DEPT_NAM2_1_0_,
department0_.MGR_ID as MGR_ID3_1_0_
from
DEPARTMENTS department0_
where
department0_.DEPT_ID=?
DEPT-BB
Hibernate:
select
manager0_.MGR_ID as MGR_ID1_2_0_,
manager0_.MGR_NAME as MGR_NAME2_2_0_,
department1_.DEPT_ID as DEPT_ID1_1_1_,
department1_.DEPT_NAME as DEPT_NAM2_1_1_,
department1_.MGR_ID as MGR_ID3_1_1_
from
MANAGERS manager0_
left outer join
DEPARTMENTS department1_
on manager0_.MGR_ID=department1_.DEPT_ID
//注意这里
where
manager0_.MGR_ID=?
MGR-BB

虽然同样查出来结果,但明显是数据巧合。这里的左外连接SQL有逻辑问题:

left outer join
DEPARTMENTS department1_
on manager0_.MGR_ID=department1_.DEPT_ID

应该是dept.manager_id = mgr.manager_id 而不是dept.dept_id = mgr.manager_id!

这里就要说到博文开头的说明:

<!-- 
没有外键的一端需要使用one-to-one元素,该元素使用 property-ref
属性指定使用被关联实体(department)主键以外的字段作为关联字段
-->
<one-to-one name="dept"
class="Department"
property-ref="mgr">
</one-to-one>

如果one-to-one节点不配置property-ref属性,默认会使用dept主键进行关联对象。而配置了该属性后,如上所示,则会使用dept.mgr在数据库对应的列(manager_id)来对应!

测试测试结果如下:

Hibernate: 
select
department0_.DEPT_ID as DEPT_ID1_1_0_,
department0_.DEPT_NAME as DEPT_NAM2_1_0_,
department0_.MGR_ID as MGR_ID3_1_0_
from
DEPARTMENTS department0_
where
department0_.DEPT_ID=?
DEPT-BB
Hibernate:
select
manager0_.MGR_ID as MGR_ID1_2_0_,
manager0_.MGR_NAME as MGR_NAME2_2_0_,
department1_.DEPT_ID as DEPT_ID1_1_1_,
department1_.DEPT_NAME as DEPT_NAM2_1_1_,
department1_.MGR_ID as MGR_ID3_1_1_
from
MANAGERS manager0_
left outer join
DEPARTMENTS department1_
on manager0_.MGR_ID=department1_.MGR_ID
where
manager0_.MGR_ID=?
Hibernate:
select
department0_.DEPT_ID as DEPT_ID1_1_0_,
department0_.DEPT_NAME as DEPT_NAM2_1_0_,
department0_.MGR_ID as MGR_ID3_1_0_
from
DEPARTMENTS department0_
where
department0_.MGR_ID=?
MGR-BB

另外需要注意的是,在查询没有外键的实体对象时, 使用的左外连接查询, 一并查询出其关联的对象并已经进行初始化。

代码如下:

@Test
public void testGet2(){
Manager mgr = (Manager) session.get(Manager.class, 1);
System.out.println(mgr.getMgrName());
System.out.println(mgr.getDept().getDeptName());
}

测试结果如下:

Hibernate: 
select
manager0_.MGR_ID as MGR_ID1_2_0_,
manager0_.MGR_NAME as MGR_NAME2_2_0_,
department1_.DEPT_ID as DEPT_ID1_1_1_,
department1_.DEPT_NAME as DEPT_NAM2_1_1_,
department1_.MGR_ID as MGR_ID3_1_1_
from
MANAGERS manager0_
left outer join
DEPARTMENTS department1_
on manager0_.MGR_ID=department1_.MGR_ID
where
manager0_.MGR_ID=?
MGR-BB
DEPT-BB

【3】基于主键映射的双向一对一

基于主键的映射策略:指一端的主键生成器使用 foreign 策略,表明根据”对方”的主键来生成自己的主键,自己并不能独立生成主键。 ​​<param>​​ 子元素指定使用当前持久化类的哪个属性作为 “对方”。

<id name="deptId" type="java.lang.Integer">
<column name="DEPT_ID" />
<!-- 使用外键的方式来生成当前的主键 -->
<generator class="foreign">
<!-- property 属性指定使用当前持久化类的哪一个属性的主键作为外键 -->
<param name="property">mgr</param>
</generator>
</id>

采用foreign主键生成器策略的一端增加 one-to-one 元素映射关联属性,其one-to-one属性还应增加 constrained=“true” 属性;另一端增加one-to-one元素映射关联属性。

<!--  
采用 foreign 主键生成器策略的一端增加 one-to-one 元素映射关联属性,
其 one-to-one 节点还应增加 constrained=true 属性, 以使当前的主键上添加外键约束
-->
<one-to-one name="mgr" class="Manager" constrained="true"></one-to-one>

constrained(约束):指定为当前持久化类对应的数据库表的主键添加一个外键约束,引用被关联的对象(“对方”)所对应的数据库表主键。

修改Department.hbm.xml如下:

<hibernate-mapping package="com.jane.one2one.primary">

<class name="Department" table="DEPARTMENTS">

<id name="deptId" type="java.lang.Integer">
<column name="DEPT_ID" />
<!-- 使用外键的方式来生成当前的主键 -->
<generator class="foreign">
<!-- property 属性指定使用当前持久化类的哪一个属性的主键作为外键 -->
<param name="property">mgr</param>
</generator>
</id>

<property name="deptName" type="java.lang.String">
<column name="DEPT_NAME" />
</property>

<!--
采用 foreign 主键生成器策略的一端增加 one-to-one 元素映射关联属性,
其 one-to-one 节点还应增加 constrained=true 属性, 以使当前的主键上添加外键约束
-->
<one-to-one name="mgr" class="Manager" constrained="true"></one-to-one>

</class>
</hibernate-mapping>

修改Manager.hbm.xml如下:

<hibernate-mapping package="com.jane.one2one.primary">

<class name="Manager" table="MANAGERS">

<id name="mgrId" type="java.lang.Integer">
<column name="MGR_ID" />
<generator class="native" />
</id>

<property name="mgrName" type="java.lang.String">
<column name="MGR_NAME" />
</property>

<one-to-one name="dept" class="Department"></one-to-one>

</class>

</hibernate-mapping>

【4】代码测试

① 持久化操作

此时先插入哪一个都不会有多余的 UPDATE,程序总会先插入Manager,因为Department主键是根据Manager主键生成的。

测试代码如下:

@Test
public void testSave(){

Department department = new Department();
department.setDeptName("DEPT-DD");

Manager manager = new Manager();
manager.setMgrName("MGR-DD");

//设定关联关系
manager.setDept(department);
department.setMgr(manager);

//保存操作
//先插入哪一个都不会有多余的 UPDATE
session.save(department);
session.save(manager);

}

测试结果如下:

Hibernate: 

create table DEPARTMENTS (
DEPT_ID integer not null,
DEPT_NAME varchar(255),
MGR_ID integer,
primary key (DEPT_ID)
) engine=InnoDB
Hibernate:

create table MANAGERS (
MGR_ID integer not null auto_increment,
MGR_NAME varchar(255),
primary key (MGR_ID)
) engine=InnoDB
Hibernate:

alter table DEPARTMENTS
drop index UK_hxcnjqu282dceadn2uqf7sc5
Hibernate:

alter table DEPARTMENTS
add constraint UK_hxcnjqu282dceadn2uqf7sc5 unique (MGR_ID)
Hibernate:

alter table DEPARTMENTS
add constraint FKmd738stvsm76ss2glhif6ljdv
foreign key (DEPT_ID)
references MANAGERS (MGR_ID)
Hibernate:

alter table DEPARTMENTS
add constraint FKh0nupqefmu52d6n1ny0vq9g9k
foreign key (MGR_ID)
references MANAGERS (MGR_ID)
Hibernate:
insert
into
MANAGERS
(MGR_NAME)
values
(?)
Hibernate:
insert
into
DEPARTMENTS
(DEPT_NAME, DEPT_ID)
values
(?, ?)

② 对象获取

测试代码一如下:

@Test
public void testGet(){
//1. 默认情况下对关联属性使用懒加载
Department dept = (Department) session.get(Department.class, 1);
System.out.println(dept.getDeptName());
// session.close();
//2. 如果session关闭则会会出现懒加载异常的问题.
Manager mgr = dept.getMgr();
System.out.println(mgr.getMgrName());
}

测试结果如下:

Hibernate: 
select
department0_.DEPT_ID as DEPT_ID1_1_0_,
department0_.DEPT_NAME as DEPT_NAM2_1_0_
from
DEPARTMENTS department0_
where
department0_.DEPT_ID=?
DEPT-DD
Hibernate:
select
manager0_.MGR_ID as MGR_ID1_2_0_,
manager0_.MGR_NAME as MGR_NAME2_2_0_,
department1_.DEPT_ID as DEPT_ID1_1_1_,
department1_.DEPT_NAME as DEPT_NAM2_1_1_
from
MANAGERS manager0_
left outer join
DEPARTMENTS department1_
on manager0_.MGR_ID=department1_.DEPT_ID
//这里注意,主键对应
where
manager0_.MGR_ID=?
MGR-DD

测试代码二如下:

@Test
public void testGet2(){
//在查询没有外键的实体对象时, 使用的左外连接查询, 一并查询出其关联的对象
//并已经进行初始化.
Manager mgr = (Manager) session.get(Manager.class, 1);
System.out.println(mgr.getMgrName());
System.out.println(mgr.getDept().getDeptName());
}

测试结果如下:

Hibernate: 
select
manager0_.MGR_ID as MGR_ID1_2_0_,
manager0_.MGR_NAME as MGR_NAME2_2_0_,
department1_.DEPT_ID as DEPT_ID1_1_1_,
department1_.DEPT_NAME as DEPT_NAM2_1_1_
from
MANAGERS manager0_
left outer join
DEPARTMENTS department1_
on manager0_.MGR_ID=department1_.DEPT_ID
// 这里主键对应
where
manager0_.MGR_ID=?
MGR-DD
DEPT-DD

参考博文:基于注解的双向一对一


举报

相关推荐

0 条评论